This invention relates to an apparatus and method for electrostatically reproducing a selected image, and in particular, relates to an electrostatic imaging system utilizing a development fluid such as liquid ink for recording an image onto a copying medium.
Fluid development of electrostatic images is not new, an early system embodying such a concept being disclosed in U. S. Pat. No. 3,084,043 issued to R. W. Gundlach on Apr. 2, 1953 and assigned to the instant assignee. Though specific prior art fluid development systems vary, a typical apparatus may include a rotatably drum having a photoconducting surface and an electrically conductive substrate. A transparent interposer may be belted between a roller and the rotatably drum thereby contacting a portion of the photoconducting surface at all times.
In such liquid development systems, a portion of the photoconducting surface, not then rotated into contact with the transparent interposer, is electrostatically charged by any suitable means. The charged portion of the photoconducting surface is then subjected to a positive or negative light and shadow representation of the image to be developed. As a result, the charged portion of the photoconductive surface is discharged through the conductive substrate at points struck by the lighted portion of the light and shadown representation of the selected image. Accordingly, the original charge applied to the photoconducting surface, upon receipt of the light and shadow representation, assumes a pattern corresponding to a positive or negative image of the image to be developed.
Electrostatic imaging systems of the prior art further include a source of development fluid, and means such as an intricately patterned gravure roll, adapted to receive metered amounts of fluid from said source. The gravure roll is typically disposed near the periphery of the rotatable drum, at a point where the transparent interposer is out of contact with the photoconducting surface. As the interposer is compressed between the gravure roll and a back-up roller, development fluid is electrostatically attracted to the portion of the interposer corresponding to the charged portion of the photoconducting surface.
The fluid-carrying interposer is then moved into contact with a copying medium such as a sheet of copy paper. A pressure roller, adapted to urge the paper against the interposer, facilitates passage of the development fluid to the paper. Since the development fluid carried on the interposer is typically in the form of the selected image, that image is faithfully transferred from the interposer to the paper. After development the photoconducting surface is discharged through the conductive substrate, thereby eliminating any residual electrostatic charge remaining on the photoconducting surface. At substantially the same time, excess development fluid on the transparent interposer is removed by means such as a doctor blade, thereby readying the apparatus for the subsequent development of other selected images.
Though fluid development systems of the type described have been used to reproduce selected images, they are subject to numerous drawbacks and deficiencies. For example, it has been found that the gravure roll adapted to transfer development fluid to the interposer is preferably fabricated from a hardened metal such as steel, brass, etc., in order to maintain the intergrity of the intricately patterned gravure surface. Structural hardness is also desirable to provide a sufficiently durable gravure. However, when the hardened metal gravure roll comes into contact with the interposer during the transfer of development fluid, damage to the interposer frequently occurs. Moreover, though the interposer tends to cushion the forces applied to the rotatable drum by the hardened gravure roll, such forces often impair the glass-like finish of the photoconducting surface. Attempts to militate against these adverse effects to the photconducting surface by providing a resilient undercoating therefor are costly and difficult to achieve.
Another drawback in the prior art fluid development systems of the type described results from the manner in which the development fluid on the interposer is transferred to the copy paper. More particularly, upon development, the copy paper tends to adhere to the fluid-carrying interposer, and thus follow the rotation of the necessarily large-diameter drum. This is, of course, undesirably since, instead of following the interposer inside the apparatus, the developed copy paper should be passed to an outlet chute where it can be retrieved by an attendant. Though mechanical means, such as pick-off fingers and the like have generally been deployed to strip the copy paper off the interposer, they increase the cost of the apparatus, and diminish reliability to the extent that they are not 100 percent effective.
As explained in greater detail, below, the apparatus and method disclosed herein overcome these and other drawbacks and deficiencies in the fluid development systems of the prior art.